docs(c4): add L3 frontend 3c/3d and sequence diagrams

docs/architecture/c4/seq-auth-flow.puml

@@ -0,0 +1,26 @@
+@startuml
+title Authentication Flow
+
+actor User
+participant Browser
+participant "Frontend (SvelteKit)" as Frontend
+participant "Backend (Spring Boot)" as Backend
+participant PostgreSQL as DB
+
+User -> Browser: Enter email + password
+Browser -> Frontend: POST /login (form action)
+Frontend -> Frontend: Base64 encode "email:password"
+Frontend -> Backend: GET /api/users/me\nAuthorization: Basic <token>
+Backend -> Backend: Spring Security parses Basic Auth
+Backend -> DB: SELECT user WHERE email=?
+DB --> Backend: AppUser + groups + permissions
+Backend -> Backend: BCrypt.matches(password, hash)
+Backend --> Frontend: 200 OK — UserDTO
+Frontend -> Browser: Set-Cookie: auth_token=<base64>\n(httpOnly, SameSite=strict, maxAge=86400)
+Browser -> Frontend: GET / (next request)
+Frontend -> Frontend: hooks.server.ts reads auth_token cookie
+Frontend -> Backend: GET /api/users/me\nAuthorization: Basic <token>
+Backend --> Frontend: 200 OK — user in event.locals
+Frontend --> Browser: Render page with user context
+
+@enduml